Power supply circuits, such as those incorporated into the electrical designs of most everyday electronic devices, including, but not limited to, televisions, television set-top boxes, audio receivers, Compact Disc (CD) and Digital Versatile Disk (DVD) players, standalone digital video record recorders (DVRs), and desktop computers, are tasked with converting an input alternating-current (AC) voltage into multiple direct-current (DC) power voltages for various components within the device. Generally, this conversion involves a two-step process, wherein the input AC voltage (typically 110 volts AC in the United States) is first converted into a single, relatively-high DC voltage, such as 15-20 volts DC, in a first conversion stage. This first stage normally employs a sizeable transformer for the storage of energy to be consumed in a second conversion stage.
In the second conversion stage, the high DC voltage is then converted into the several low-level DC power voltages required by the device circuitry, such as, for example, 3.3 volts DC, 2.5 volts DC, 1.8 volts DC, and 1.2 volts DC, due to the varied power supply requirements of the circuits typically utilized in the device. In many cases, one low-level DC output voltage, such as a 5.0 volt DC output, is employed to generate the other lower-level DC output voltages. In such situations, the power supply may exhibit a negative resistance effect, in which the low-level DC output voltage cannot provide the peak current required for the other output voltages. To counteract this effect, the power supply normally employs a significant amount of capacitance at the output of the first stage, and possibly at the outputs of the second stage, so that the peak current requirements of the all of the output voltages are satisfied.
Normally, the output levels of the various DC output voltages must be maintained within a relatively narrow voltage range to allow the components employing these voltages to operate properly. To address this requirement, the power supply circuit oftentimes is designed to monitor the output of the high-level DC voltage level, and alter the operation of the portion of the power supply circuit performing the first level of voltage conversion as a result of the monitoring operation in order to maintain an acceptable voltage level for the high-level DC voltage and the subsequent lower DC voltages used in the device.
In some power supplies as described above, an additional circuit stage is utilized for power factor correction. This stage normally takes the form of a boost “chopper” or converter used to regulate the output voltage, as well as to ensure that the output current is aligned in-phase with the output voltage. As boost converters are prone to produce electromagnetic interference (EMI), a two-stage EMI filter is normally incorporated into such a power supply to reduce any possible high-ripple current at the input of the boost converter and thus diminish EMI. In many low power conversion power supplies, a flyback converter is utilized at the output stage to provide a cost-effective solution that also improves the power factor of the power supply.